The Collaborative International Dictionary of English v.0.48:

Hobo \Ho"bo\, n.; pl. Hobos or Hoboes. [Of uncertain
   origin.]
   A professional tramp; one who spends his life traveling from
   place to place, esp. by stealing rides on trains, and begging
   for a living. [U. S.] -- Ho"bo*ism, n.

   Syn: tramp; bum; vagrant; knight of the road.
        [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

hobo
    n 1: a disreputable vagrant; "a homeless tramp"; "he tried to
         help the really down-and-out bums" [syn: tramp, hobo,
         bum]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "hobo":
   Arab, Bowery bum, alien, beach bum, beachcomber, beggar,
   beggarly fellow, blighter, bo, budmash, bum, bummer, cadger,
   caitiff, case, character, crackpot, crank, derelict, devil, dogie,
   drifter, drunkard, eccentric, fanatic, floater, gamin, gamine,
   good-for-naught, good-for-nothing, guttersnipe, hermit,
   homeless waif, human wreck, idler, kook, landloper, lazzarone,
   loafer, lone wolf, loner, losel, lowlife, mauvais sujet, maverick,
   mean wretch, mendicant, mendicant friar, mendicant order,
   meshuggenah, moocher, mucker, mudlark, natural, no-good,
   nonconformist, nut, odd fellow, oddball, oddity, original,
   outsider, panhandler, pariah, pauvre diable, piker, pilgarlic,
   poor creature, poor devil, queer duck, queer fish, queer specimen,
   ragamuffin, ragman, ragpicker, rara avis, rounder, sad case,
   sad sack, schnorrer, screwball, scrounger, ski bum, skid-row bum,
   solitary, stiff, stray, street Arab, street arab, street urchin,
   sundowner, surf bum, swagman, swagsman, tatterdemalion, tennis bum,
   tramp, truant, turnpiker, type, urchin, vag, vagabond, vagrant,
   vaurien, waif, waifs and strays, wastrel, worthless fellow, wretch,
   zealot